These cases require us to decide whether a plan to dispose of mining waste violates the Clean Water Act.

The petitioner Coeur Alaska owns Kensington Gold Mine in Southeast Alaska. When it opens and begins its operation, extracting the gold will produce several million tons of crushed rock mixed with water. This mixture is called slurry and its waste and it must be put somewhere.
Coeur Alaska plans to put the slurry in a small lake near the mine. The slurry will kill the lake’s fish and plant life. After the mine closes, however, coeur Alaska will cover the lake bed with soil and thereby reclaim it. The broader and shallower lake will sustain at least as much aquatic life as it does now.
If the slurry did not go in the lake, Coeur Alaska will propose to put it on nearby wetlands destroying them permanently.

Coeur Alaska obtained approval for its plan from two agencies that administer the Clean Water Act, the Army Corps of Engineers and the Environmental Protection Agency or the EPA.
The Corps of Engineers acts under Section 404 of the Act and EPA acts under Section 402 of the Act.
Acting under Section 404, the Corps of Engineers issued Coeur Alaska a permanent to discharge the slurry into the lake. And acting under Section 402 of the Act, the EPA issued Coeur Alaska a permit to discharge water from the lake into a downstream creek.

Respondent’s three environmentalist groups led by the Southeast Alaska Conservation Council were SEACC sued the agencies in federal court.

SEACC made two arguments.

First, SEACC argued that to put its slurry in the lake in the first place, Coeur Alaska requires a permit from the EPA not the Corps of Engineers.

Now, the EPA regulation is called performance standard. It forbids the discharge of waste from gold mine like Coeur Alaska’s.

The Court of Appeals agreed with both SEACC’s arguments and it vacated the Corps of Engineers’ permit.

We granted certiorari and we now reverse.

We disagree with both SEACC’s arguments.

First, the Clean Water Act states the Corps of Engineers and not the EPA, is the proper agency to issue a permit for the slurry discharge. This is because slurry is defined by the regulation as fill material. In Section 404 of the Act authorizes the Corps to permit discharges of fill material. Section 402 by contrast states that the EPA may not issue permit for these discharges.

Second, SEACC argues that EPA’s performance standards forbid the slurry discharge. Both agencies disagree with that position however. In the view of both agencies, the EPA’s performance standard does not apply to discharges of fill material because the statute and regulations do not give a clear answer to this question, we defer to the agencies’ reasonable interpretation.

Five factors show that the agencies’ interpretation is reasonable.

First, the agencies interpretation preserves the rule for the EPA’s performance standard. That standard continues to apply to the discharge of water from the lake into the downstream creek.

Second, the agencies’ interpretation does not permit discharges to avoid performance standards by characterizing small amounts of solid waste as fill material.

Third, the agencies’ interpretation allows the Corps of Engineers to determine what is best for the aquatic environment. Here, for example, the Corps determined that the slurry discharge though harming the lake in the short-term will be better for the environment in the long-term than with the alternative of permanent loss of wetlands.

Fourth, the agencies agreed that no toxic pollutants maybe discharged as fill material.

And fifth, SEACC has not offered an alternative interpretation that harmonizes the statute and regulations.

The judgment of the Court of Appeals is reversed and these cases are remanded for proceedings consistent with this opinion.